1. Field of the Invention
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, and other glossy media; and more particularly to apparatus and methods that construct text or images from individual marks created on the printing medium, in a two-dimensional pixel array, by a pen or other liquid-ink-ejecting marking element or head that scans across the medium.
The invention is particularly beneficial in printers that operate by an inkjet process. That process discharges individual ink drops onto the printing medium.
2. Related Art
U.S. Pat. No. 5,065,169, of Vincent et al., introduces the importance of controlling pen-to-printing-medium distance, and flatness of the medium, in an inkjet printer. Vincent discloses one way of performing those functions by means of a spacer formed as a skid, roller or the like that travels with the pen.
Systems following Vincent addressed a more-difficult problem of performing like functions in staggered-multiple-pen systems--where traveling skids or rollers were problematic due to the tendency of the roller or skid on a trailing pen to smear the ink laid down by a leading pen. These later systems provided stationary hold-down plates, sometimes called "shims", intended to restrain the print medium against print-medium deformation such as curl or cockle.
Such systems were not fully effective in controlling such deformation. As a result those systems were sometimes subject to print-medium crashes, particularly near the leading edge of each sheet, and degraded image quality due to curling and other flight-time-related errors--particularly along the lateral edges over the full height of each sheet.
These limitations were especially noteworthy in conjunction with use of ink-drying heaters and ink-spray-removal blowers--modern developments whose objects are to reduce drying time for nonabsorbent media and to carry away waste ink spray, but which unfortunately have some tendency to aggravate curl and cockle. Waste ink is common in inkjet systems in the air above the print medium, whether persisting as minute spray droplets or present as recondensable vapor, and is advantageously carried off gently in an exhaust air stream to avoid its deposition onto the printing medium.
The above-mentioned related patent document of Broder et al. heralded a significant improvement in hold-down-plate arrangements for controlling pen-to-print-medium distance and print-medium flatness. The Broder system accommodates operation of staggered multiple pens, without smearing of leading-pen marks by trailing pens. It also accommodates such pens when used to print on glossy media, and with a print-zone heater and spray-removal blower, while controlling print-medium deformation and avoiding so-called "paper crashes".
Broder et al. obtained these improvements by introducing a dual guide system of central and lateral hold-down plates that restrain the print medium in a coordinated way. The central plate is upstream from the pen or pens, in part angled longitudinally down into contact with the print medium, and extends laterally across the width of the medium except in one or more regions that are laterally near the engagement of a print-medium advancing device.
The other, lateral, guide system is positioned laterally outboard from the pen, and extends laterally across the medium only in one or more regions laterally near the engagement of the advancing device. These "one or more regions" preferably are only near the lateral edges of the medium.
Thus the two guide systems complement each other in function. The first guide restrains the medium over an area that stops short of the lateral edges of the medium; and the second is preferably bifurcated and disposed laterally in two directions from the pen, restraining the medium across only its lateral edges.
Despite these very favorable developments due to the teachings of Broder et al., certain imperfections have been noted in the quality of documents printed with the described system. In particular, spurious ink marks occasionally appeared in the printing--often transverse straight lines, generally at various regions of the printing-medium width and in the color of whatever ink was being used in the printer.
The appearance of these linear markings was erratic in time, and seemed to neither correlate with any feature of the document being printed nor exhibit the characteristic pixel structure of the inkjet printing process. The marks, however, were very evidently associated with quantities of liquid ink seen on the top surface of the horizontal panel 121h (FIGS. 8 and 9) and angled panel 121a of the central hold-down plate 121, and ink I which was seen running down the angled parts 121a of its top surface toward the straight edge 121e of the plate, and indeed on the edge 121e--from which ink was being transfered from the edge to the printed sheets 30. The upper one of the angled panels 121a is particularly steep, about twenty-five degrees from horizontal.
In the earlier configurations in which these phenomena were observed, the guide plate 121 was mounted above an adjacent plenum cover 50 (in this document not distinguished from the plenum generally) and pressed very lightly though firmly on the printing medium 30. The guide plate 121 included a generally horizontal panel 121h, and downward angled panels 121a--terminating in a straight, blunt edge 121e.
A rear panel 121r served to stiffen or rigidify the guide plate 121. A section 50r of the plenum cover 50, roughly several centimeters from the print zone Z, was bent upward to help hold thin plastic paper guides (not shown) below the cover, leaving a narrow window 50w in the plenum cover 50.
The medium 30 advanced (from left to right in FIGS. 8 and 9) above a supporting grill 73 with orifices 74 and beneath the pens 10--while the pens 10 traversed along a direction which in FIG. 8 is in and out of the plane of the paper. Nozzles 11 forming part of each pen 10 discharged ink toward the top surface of the print medium 30 in the print zone Z.
A preheater 78' and a heater 78 shown schematically to the left of and below the supporting grill 73 were provided to predry the print medium and to speed drying of ink deposited on the printing medium 30--to form a desired image. Air was moved slowly and gently from right toward left (as drawn in FIGS. 8 and 9) to carry away waste ink spray as droplets, and possibly components of the ink formulation in vapor form. This air movement was provided by a blower--not shown, but along a path that begins with the plenum structure 50 at left in the drawings--which sucked air 161 away from the unprotected and already-imprinted regions of the print medium, across 162 the print zone Z and into the plenum 165 toward 168 the blower.
In relation to the direction 33 of print-medium 30 movement, the hold-down guide 121 was at all times upstream from the pens 10. Thus it was not physically possible for the guide 121 to pick up ink by brushing it from the medium 30; and there was no path for ink migration directly from pen reservoirs to the guide plate 121.
The source of this ink deposit was accordingly recognized as precipitation of waste ink from the air in the print zone Z. In a representative inkjet printer, as outlined above, a blower moves that air slowly away from the print zone Z to prevent significant amounts of the spray from falling, precipitating or otherwise being deposited onto the sheet being printed.
This ink-spray-transporting air flow is preferably routed away from unprotected areas of the document--where the waste spray might settle onto those areas and so spoil the printed image. No protective structure is present or desirable in the downstream direction, which is to say the direction 33 of print-medium 30 advance; but the hold-down plate 121 offers to serve as a shield over the print medium in the opposite, upstream direction.
It is for this reason that advantageously the drying-air flow 161-168 is made to flow in that opposite direction, toward 162 and past (above) 165 the hold-down plate 121--or in other words counter to the direction 33 of print-medium 30 movement.
As can now be appreciated, this configuration was originally thought to have disposed of the waste-ink spray with some finality. In the more-demanding environment of close curl/cockle control and high image throughput, however, the same waste spray can come back to haunt the system.
The plate 121, while thus upstream along the direction 33 of printing-medium 30 movement, is downstream along the direction 161-168 of air movement created by the drying blower. Hence the waste ink spray moves toward, and should move over and past, the plate 121--into the blower plenum structure 50 where it can harmlessly precipitate or be filtered out of the air stream, or both.
As will now be understood, the above-mentioned ink deposits, ink flow I, and undesirable markings all resulted from failure of some of the waste ink to pass completely over and beyond the guide plate 121.
Less clear, heretofore, was why this ink was building up where it was, on the plate 121 immediately adjacent to the print zone Z; and what could be done to prevent it from doing so or neutralize its effects; and how such preventive measures might be implemented. As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement.